MEMS devices are a combination of micro mechanical and micro electronic systems that are revolutionizing nearly every product category by bringing together these systems. A MEMS device typically comprises a movable micro mechanical structure and silicon based microelectronics. One type of known MEMS device is a MEMS transducer. Analog Devices™ manufactures a capacitive MEMS transducer used in an air-bag system for crash detection. For example, a crash is detected by monitoring the movement of the mechanical structure in the MEMS transducer using associated microelectronics in the MEMS transducer. The mechanical structure in the transducer comprises a capacitive plate which is relative to another capacitive plate in the transducer. As the mechanical structure moves, a change in capacitance is caused by the displacement of the capacitive plate. This change in capacitance is detected by the microelectronics and used to activate the air bag.
The Analog Devices™, MEMS transducers are manufactured from a single wafer. The mechanical structure of these transducers is created by depositing a poly-silicon layer on a silicon wafer, which is typically only a few microns thick. Because of the limited thickness of the mechanical structure, the mechanical structure suffers from performance limitations. For example, because of the minimal thickness of the mechanical structure it is difficult to restrict the movement of the mechanical structure to the desired plane. This results in the microelectronics detecting movement in the desired plane as well as movement out of the desired plane (i.e., crosstalk). Due to crosstalk, resulting from the single-wafer MEMS structure, these transducers have very limited application.
Other conventional MEMS devices typically comprise two chips wired together. In these MEMS devices, one chip includes the micro mechanical structure and the other chip includes the micro electronic structure. These two chips are manufactured separately and wired-bonded together. This results in performance degradation and increased costs. For example, stray capacitances are introduced due to the necessity of wire-bonding the two chips together. Also, these two chips must be packaged together as a single device, which increases costs.